Process for the after-treatment of phosphate-coatings

ABSTRACT

A PROCESS FOR THE AFTER-TREATMENT OF PHOSPHATIZED METAL ARTICLES, WHICH PROCESS COMPRISES TREATING THE PHOSPHATIZED SURFACE OF THE METAL ARTICLE WITH AQUEOUS SOLUTION CONTAINING AN ALKALI METAL SALT OF A NOVOLAC PHENOL-FORMALDEHYDE RESIN HAVING A MOLECULAR WEIGTH OF FROM ABOUT 300 TO ABOUT 5,000, AND HEATING THE TREATED ARTICLE AT A TEMPERATURE OF AT LEAST ABOUT 190*C. IN THE PRESENCE OF AN OXYGEN-CONTAINING ATMOSPHERE. THE ARTICLES SO TREATED ARE RENDERED EXCEPTIONALLY RESISTANT TO CORROSION AND TO UNDERCUTTING WHEN COATED WITH FINAL FINISH COATINGS.

3,697,331 PROCESS FOR THE AFTER-TREATMENT F PHOSPHATE-COATINGS Malcolm H. Shatz, Williamsville, N.Y., and Edward Leon, Parkersburg, W. Va., assignors to Hooker Chemical Corporation, Niagara Falls, N .Y. No Drawing. Filed Dec. 28, 1970, Ser. No. 102,060 Int. Cl. C23f 11/00 US. Cl. 1486.15 R 14 Claims ABSTRACT OF THE DISCLOSURE A process for the after-treatment of phosphatized metal articles, which process comprises treating the phosphatized surface of the metal article with aqueous solution containing an alkali metal salt of a novolac phenol-formaldehyde resin having a molecular weight of from about 300 to about 5,000, and heating the treated article at a temperature of at least about 190 C. in the presence of an oxygen-containing atmosphere.

The articles so treated are rendered exceptionally resistant to corrosion and to undercutting when coated with final finish coatings.

This invention relates to the chemical treatment of metal articles. More particularly, this invention relates to the chemical treatment of metal articles in such a manner as to render the metal articles exceptionally corrosion-resistant, better suited to retain final coatings, and resistant to undercutting when final finish coatings are so applied.

It is well known that bonding coatings are desirable on metal articles to protect articles from corrosion and to form a base to promote adherence to such surfaces of organic finishes such as paints, enamels, lacquers, siccative coatings and the like. A variety of processes have been developed for this purpose and many specifically different compositions have been proposed as satisfactory for use in such processes.

Presently, the majority of bonding coatings techniques entail the use of phosphate solutions or phosphatizing solutions. Such phosphatizing solutions as have been used for the purpose of inhibiting the corrosion of such as iron and steel surfaces result in phosphate coatings which, however, do not provide sufficient protection against corrosion of the metal in many cases. In order to obtain effective protection against corrosion the phosphatizing process has been followed, in the past, in most instances by rinsing with dilute chromate solutions.

However, this chromate after-rising or after-treatment of the phosphate coating has given rise to, among other things, serious disposal problems relative to the resulting Waste water. Because of the toxicity of the hexavalent and trivalent chromium, these materials must be removed almost quantitatively from the resulting waste water prior to disposal. Such recovery processes as must be practiced render the chromate rinse economically less desirable. Further, the chromic acid concentrates used for preparing and for replenishing the rinsing bath present additional difficulties in handling due to their strongly corrosive properties. The use of chromate rinses, in some instances, also gives rise to other practical difficulties. During some applications, excessive amounts may tend to either dis color or to bleed through subsequently applied top-coats. In other applications, the solutions may also tend to run or pool in confined areas of the metal article workpiece, which may cause subsequent loss of top-coat adhesion when the completed article is exposed to high humidity United States Patent 0 3,697,331 Patented Oct. 10, 1972 conditions. Because of the running and pooling of the chromate rinsing solution, certain areas of the metal articles are inherently left deficient in rinse treatment. Such areas exhibit as a result poor corrosion-resistance to undercutting when exposed to corrosive atmospheres.

Attempts to eliminate or to modify the sealing chromium rinse by post-treating the phosphatjzed metal articles with solutions of such coating materials as alkene phosphinic acids, acrylics, epoxies and the like have not resulted in the production of entirely satisfactory corrosion-resistant metal articles, generally, having the anti-corrosive and paint-adhesion properties attributed to chromium-rinsed phosphatized metal substrates.

It is an object of the present invention to provide for protective coatings for corrodible metal articles.

There is a further object of the present invention to provide protective coatings for corrodible metal articles which are receptive to retaining final finish coaitngs.

It is yet another object of the present invention to provide a chemically reactive metal-treating composition which will, when applied to phosphatized metal articles, render such metal articles exceptionally corrosion resistant.

These and other objects will become readily apparent in view of the following description of the present invention.

According to the present invention, an anti-corrosive and paint receptive coating is formed on a metal article by the process comprising: (a) coating the metal article with a conventional phosphatizing solution; (b) treating the phosphatized surface thereof with an aqueous solution of an alkali metal salt of a novolac phenol-formaldehyde resin having a molecular weight of from about 300 to about 5,000; and (0) thereafter heating the treated article at a temperature of at least about 190 C. in the presence of an oxygen-containing atmosphere.

The use of phenolic resins on metal to prevent corrosion and to provide a measure of protection to metal substrates is well-known and established in the art. However, such value has been achieved by the use of very high molecular weight phenolic resins forming relatively thick, contiguous films, the molecular weight of such phenolic resins varying from 20,000 to greater than 100,000.

It has now been found that very low molecular weight alkali metal novolac phenolic resins, applied to phosphatized metal surfaces in amounts of from about 4 to about 400 milligrams of resin per square foot of surface area, and generally from about to about 400 milligrams per square foot and subsequently heated to temperatures in excess of about *C. in the presence of an oxygen-containing atmosphere provide for remarkably superior corrosion protection and other performance properties, and do not behave as barrier layers, as do the high molecular weight phenolic resins currently used in the art. Apparently, when the alkali metal salts of novolac phenolic resins are heated to temperatures in excess of about 190 C. in the presence of oxygen, the resins tend to exhibit chemical, rather than contiguous, behavior. It is believed that, at temperatures in excess of about 190 C., the low molecular weight novolac resin alkali metal salts complex and react with the Zinc phosphate crystals and iron oxides present to form caustic-insoluble complexes, and that it is this caustic-insolubility of the formed complexes which provides for the excellent undercutting and corrosion resistance.

The alkali metal salt of novolac phenol-formaldehyde resins which have been employed with success in the process of the present invention are those resin salts which may be generally. depicted as having the formula:

wherein Me is an alkali metal, In is an integer having a value of from about 1 to about 45 and n is an integer having a value of from about 1 to 15. Such phenol-formaldehyde resins as represented by 'Formula I, designated as orthonovolac, are substantially phenol-terminated chain polymers in which the phenolic nuclei are united by methylene bridges, for the most part located ortho to the phenolic hydroxyl groups. Those resins represented by Formula II, designated as random novolacs, are substantially phenoleterminated chain polymers in which the phenolic nuclei are united by methylene bridges, for the most part located ortho and para relative to the phenolic hydroxyl groups.

Such resins are permanently soluble and thermoplastic and cure to insoluble, infusible products upon the application of heat, or upon the addition thereto of a source of formaldehyde.

The resins are of-adequate hardness with a softening temperature of from about 50 to about 110 C., to permit grinding thereof, exhibit good curing properties'including a fast rate of cure, and attain a high degree of strength upon being cured.

Any of the several alkali metals may be employed in producing the novolac resin salts; however, it is preferred to use either sodium or potassium as the alkali metal. Sodium is the most preferred, and the examples hereinafter set forth so indicate.

The alkali metal novolac phenolic resins are those generally produced by reacting an alkali metal hydroxide, preferably sodium hydroxide, with novolac resins in water solutions on a 1:1 molar ratio, based on the. phenolic ,hydroxylcontent of the resin, at temperatures on the order of from about 25 to about 75 C. Less alkali metal hydroxide may be used, depending upon the degree of salt formation desired; however, suflicient alkali metal hydroxide isrequired to render the novolac resin water-soluble. Phosphatized metal articles have been successfully treated by using resin solutions produced by reacting sodium hydroxide: phenolic hydroxyl on a 0.5: 1, ratio and subsequently heating the treated articles at temperatures greater than about 190 C. in the presence of an oxygencontaining atmosphere.

The novolac resins can be prepared by any one of the number of suitable methods known in the 'art, such as that disclosed in US. Pats. 2,475,587 and 3,108,978, the teachings of which are herein incorporated by reference.

Generally acceptable formulations for the purpose of the present invention are those aqueous ortho novolac or random novolac resin salt solutions wherein the resin content ranges from about 0.25 to by weight and the alkali metal: phenolic hydroxyl ratio is from about 0.5:1 to about 1:1. Preferably, those aqueousso'lutions containing from about 0.25 to 5.0 percent by weight resin having an alkali metal: phenolic hydroxyl ratio of from 0.75 :1 to about 1:1 are employed.

The aqueous alkali metal novolac treating solution may be applied to the phosphatized metal article either by OMe OMe

spraying or by immersing the article into the solution, or by any other suitable means, to deposit resin in an amount of less than about 400 milligrams per square foot on the phosphatized metal surface. After the article has been treated, the solvent is removed by any suitable means and the article is heated at temperatures on the order of about 190 C. to about 270 C. in the presence of oxygen for periods sufiicient to produce a finished treatment, generally from about 1-5 seconds to about 5 minutes. Prefera'bly, heating is effected at temperatures on the order of from about 190 to 205 C. for periodsof about to about seconds, in the presence of an oxygen-containing atmosphere. Generally, at a temperature of about 205 0., treatment is completed in less than aboutv 180 seconds.

The time required to effect the heating step is, of course, dependent upon the makeup of the resin treating composition, the temperature employed and the nature of the article being treated.

In another variation of the present process, the phosphatized metal article is immersed in the alkali metal novolac solution, removed and heat treated, then subjected to a final water rinse.

The surface of the metal article, following the treatment, is highly adherent, receptive to and retentive to the application of paints, varnish, and lacquers. Thus, metal articles, pretreated according to the process of the present invention may receive organic finishes having an end use as automobile finishes, exterior trim, coatings on metal building panels, appliance coatings, metal furniture coatings and the like.

Metal surfaces which may be treated according to the process of the present invention include the ferrous metals, aluminum, galvanized metals, alloys and the like.

The deposition of the phosphate coating may be effected by any of the conventional phosphatizing procedures. Aqueous phosphatizing solutions are generally prepared by dissolving in water small amounts of phosphoric acid and a metal salt such as a nitrate, phosphate, nitrite, sulfate, chloride or bromide of sodium, manganese, zinc, cadmium, iron, nickel, lead, copper or aluminum. Preferably the metal salt used is one which, in aqueous solution will yield zinc or iron ions. An oxidizing agentsuch as sodium chlorate, sodium perborate, sodium nitrite, sodium chloride, potassium perchlorate, or hydrogen peroxide can be included in the phosphatizing solution to depolarize the metal surface being treated and thereby increase the rate at which the phosphate is formed on the metal article.

Other auxiliary agents such as anti-smudging agents, coloring agents, metal cleansing agents and the like may also be incorporated in the phosphatizing solution. A common type of commercial phosphatizing bath which contains zinc ions, phosphate ions, and a depolarizer is made by dissolving small amounts of zinc phosphate, sodium nitrate, and phosphoric acid in water.

The phosphatizing solutions suitable for use in the process of the present invention can also be conveniently formulated by first dissolving zinc nitrate and the like in sufiicient water to yield the desired concentration of zinc ions and thereafter adjusting the acidity with phosphoric acid. Another type of commercial phosphatizingbath is one containing nitrate ions, zinc ions, phosphate ions and either nitrite or chlorate ions as a depolarizer.

However, the present process is not dependent upon the use of a particular phosphatizing solution, and any of the several commercial aqueous phosphatizing solutions commonly used in the art may be employed to phosphatize the surfaces of metal aritcles to be treated according to the present process. Preferred commercial phosphatizing solutions are those which, on being deposited on metal substrates, will yield zinc or alkali metal phosphates.

In the accepted practice of phosphatizing a metal article, the surface thereof is usually cleansed by physical and/or chemical means such as immersion in or spraying with an aqueous caustic cleanser, chemical abrading or polishing, vapor degreasing and the like. For the purpose of the present process, it is preferred to cleanse the metal article to be phosphatized and subsequently treated with the novolac treating solutions by immersing the article for a period of about 60 seconds in a commercial aqueous caustic metal cleanser at a temperature of about 60 C. Following the cleansing of the metal article, the article is generally rinsed with hot water to prevent contamination of the phosphatizing solution.

The application of the phosphatizing solution to the metal article prior to the application of the alkali metal novolac resin treating solution may be accomplished by any suitable means known in the art. Because the phosphatizing solutions are generally applied at temperatures of on the order of from about 50 to about 85 C., sprayphosphatizing techniques are suitable. Immersion techniques are also well-adapted to provide phosphate coatings on metal articles to be subsequently resin treated. The manner of application, however, is not important in that quiescent baths can be used, or continuous baths may be advantageously employed.

The aqueous phosphatizing solution is generally maintained at temperatures on the order of from about 50 to about 85 C. Preferably, temperatures on the order from 60 to about 70 C. are employed.

Satisfactory phosphate coatings, when applied in accordance with the procedures previously detailed, result within about 60 seconds. Longer or shorter times may be realized by varying the concentration of the phosphatizing solution and the temperature.

Upon completion of the phosphatizing step, the phosphatized metal article is rinsed with cold water and treated for a period from about 30 to about 90 seconds with the aqueous alkali metal novolac salt resin solution at a temperature of from about 25 to about 75 C.

The metal article may then be treated in a number of ways. It can be heated directly at temperatures of at least about 190 C. in the presence of an oxygen-containing atmosphere. Alternately, the article can be drained for a period of from about 60 seconds to 5 minutes, rinsed in either tap water or deionized water, and heated at temperatures in excess of about 190 C. in the presence of an oxygen-containing atmosphere. Still further the article can be subjected to rinsing and re-drying following the heating step.

Following the resin treatment of the metal articles in the following examples, each article was topcoated and then subjected to 5 percent salt spray and physical tests. The salt spray test is the American Society for Testing and Materials (ASTM) test B117-61 with painted panels scribed as given in ASTM test D-165 461. This employs a 5 percent sodium chloride fog or spray. The ratings given depend on the creepage from the scratch, given in of an inch. Ratings given as spt (S) indicate no creepage except in a small area. In the salt spray test, unless otherwise indicated, the exposure time was 120 hours. In the physical test, adhesion was determined by knife blade and the results are reported on the scale of 0 to 10, where is excellent, 8 is good, 6 is fair, 4 is poor, 2 is very poor and 0 is complete loss of adhesion.

The following examples serve to illustrate the present invention but are not to be construed as limiting it thereto.

EXAMPLE 1 A polished steel panel is cleansed for 60 seconds in a commercial hot caustic cleanser, rinsed with hot water and then immersed for 180 seconds in a commercial aqueous zinc phosphate phosphatizing solution. The coated steel panel is removed, rinsed with cold water, and thenimmersed for 60 seconds in a 0.5 percent by weight aqueous sodium ortho-novolac phenol-formaldehyde resin treating solution, the novolac resin component having a molecular Weight of about 1000 and a sodium; phenolic hydroxyl ratio of 0.75:1. The temperature of the resin treating solution is maintained at 50 C. during the immersion. The panel is then removed to an air circulating oven and heated at 205 C. for 150 seconds. Following the heating step, the treated panel is then re-rinsed with deionized water and dried at 160 C. for '60 seconds. The treated panel is painted with a commercial melamine alkyd enamel. The panel has a knife blade adhesion rating of 10, an undercutting of less than $6 of an inch and a paint retention of greater than percent.

EXAMPLE 2 A polished steel panel is cleansed for 60 seconds in a commercial hot caustic metal cleanser, rinsed with hot water and exposed to a spray of commercial aqueous zinc phosphate phosphatizing solution for 60 seconds. The coated steel panel is removed, rinsed with cold water, and immersed for 60 seconds in a 0.25 percent by weight aqueous sodium ortho-novolac phenol-formaldehyde resin treating solution, the novolac resin component having a molecular weight of about 500 and a sodium; phenolic hydroxyl ratio of about 1:1. The temperature of the resin treating solution is maintained at 50 C. during the im mersion. The panel is then removed to an air circulating oven and heated at 205 C. for seconds and then rerinsed. The treated panel, when topcoated with a commercial melamine alkyd enamel, has a knife blade adhesion rating of 10, an undercutting of less than A of an inch, and ,a paint retention of greater than 95 percent.

EXAMPLE 3 The procedure of Example 1 is observed, with the exception that the treated panel is topcoated with commerical vinyl paint. Similar excellent results obtain as in Example 1.

EXAMPLE 4 The procedure of Example 1 is observed with the exception that the treated panel is topcoated with commerical acrylic paint. Similar excellent results obtain as in Example 1.

EXAMPLE 5 The procedure of Example 1 is observed with the exception that the sodium: phenolic hydroxyl ratio is about 1:1, the resin treating solution contains 5 percent by weight resin and heating is effected at 205 C. for 60 seconds. The topcoated panel has a knife blade adhesion rating of 10, an undercutting of less than of an inch and a paint retention of 99 percent.

EXAMPLE 6 The procedure of Example 5 is observed with the exception that, following the heating step, the panel is rinsed with distilled water and heated at C. for 60 seconds. Similar excellent results obtain as in Example 5.

EXAMPLE 7 The procedure of Example 5 is observed with the exception that the treated article is heated at 190 C. for 180 seconds. Similar excellent results obtain as in Example 5.

EXAMPLE 8 The procedure of Example 7 is observed with the exception that the metal panel is subjected to a distilled water rinse following the treatment with the phosphatizing solution. Similar excellent results obtain as in Example 5.

EXAMPLE 9 The procedure of Example 7 is observed with the exception that, following the heating step, the panel is rinsed with distilled water and heated at 160 C. for 60 seconds. Similar excellent results obtain .as in Example 5.

7 EXAMPLE 10 The procedure of Example 1 is observed with the exception that, following the application of the treating resin, the panel is allowed to stand at ambient temperature for a period of 2 minutes, rinsed with distilled water and heated at 190 C. for a period of 90 seconds. Similar excellent results obtain as in Example 1.

EXAMPLE 1 1 The procedure of Example 10 is observed with the exception that the treated panel is heated at 190 C. for 150 seconds. Similar excellent results obtain as in Example 1.

EXAMPLE 13 The procedure of Example 10 isobserved with the exception that the treated panel is heated at a temperature of 205 C. for a period of 90 seconds. Similar excellent results obtain as in Example 1.

EXAMPLE 14 The procedure of Example 13 is observed with the exception that the treated panel is heated for a period of 120 seconds. Similar excellent results obtain as in Example 1.

EXAMPLE 15 The procedure of Example 13 is observed with the exception that the treated panel is heated for a period of 150 .seconds. Similar excellent results obtain as in Example 1.

EXAMPLES 16-21 The procedures of Examples 10-l5 are observed with the exception that, in each instance, the panel is allowed to stand at ambient temperature for minutes. Similar excellent results obtain as in Example 1.

Similar excellent results may be obtained by varying the resin concentration from 0.25 to percent by weight, the molecular weight of the resin, both ortho and random novolacs, from about 300 to about 5000, the heating step from about 190 to about 270 C. for periods of from about 60 to about 180 seconds, and the alkali metal: phenolic hydroxyl ratio: from about 0.5 :1 to 1:1 in various suitable combinations. A typical treatment utilizing a random novolac is shown in Example 22.

EXAMPLE 22 A polished steel panel is cleansed for 60- seconds in a commercial hot caustic metal cleanser, rinsed with hot water, and 'sprayed with a commercial zinc phosphate phosphatizing solution .for 60 seconds. The coated panel is then rinsed with cold water and immersed for 60 seconds at 50 C. in a 0.5 percent by weight aqueous solution of the sodium salt of a commercial acid-catalyzed random novolac resin having a molecular weight of about 500 and a sodium: phenolic hydroxyl ratio of about 1:1. The panel is then removed, heated at 205 C. for 150 seconds in an air circulating oven, re-rinsed with de-ionized water and dried at 160 C. for 90 seconds. The panel, when top-coated with commercial melamine alkyd enamel, exhibits similar excellent properties as those obtained in Example 1.

What is claimed is:

1. A process for the production of an, improved phosphatized metal article which comprises treating said phosphatized metal articlewith an aqueous solution consisting esentially of water and an alkali metal salt of a novolac phenol-formaldehyde resin, said resin having a molecular has molecular weight of about 1000 and an alkali metal:

phenolic hydroxyl ratio of about 1:1.

4. A process as defined by claim 1 wherein the resin *is a random novolac resin.

5. A process as. defined by claim 4 wherein the resin has a molecular weight of about 1000 and an alkali metal: phenolic hydroxyl ratio of about 1:1.

6. A process as defined in claim 1 wherein the alkali metal is sodium, the sodium: phenolic hydroxyl ratio is from about 0.5:1 to 1:1 and the novolac resin has a molecular weight of about 1000.

7. A process as defined by claim 1 wherein the treated articles are heated at a temperature of from about 190 to about 270 C. for a period of from about 30 to about 180 seconds.

8. A process as defined by claim 1 wherein the treated metalarticle is subjected to draining for a period of from about 1 to about 5 minutes following the application of the resin treating solution thereto, rinsed with water, and baked at a temperature of about 205 C. for a period of seconds in the presence of an oxygen-containing atmosphere.

9. A process as defined by claim 1 wherein the treated metal article is re-rinsed following the heating thereof, and dried at a temperature of about C.

10. A process for improving the corrosion-resistance and paint-adherence of a phosphatized metal article which comprises treating said metal article, for a period of from about 30 to about 90 seconds at a temperature of from about 25 to about 75 C., with an aqueous solution consisting essentially of water and from about 0.25 to about 10 percent by weight of an alkali metal salt of a novolac phenol-formaldehyde resin having a molecular weight of from about 300 to about 1,000, said resin having an alkali ..metal: phenolic hydroxyl ratio of from about 0.5 :l to

about 1:1, and thereafter heating the treated article at a temperature of from about 190 to about 270 C. for a period of from about 30 to about seconds in the presence of an oxygen-containing atmosphere.

11. A process as defined by claim 10 wherein the resin is an ortho-novolac resin.

12. A process as defined by claim 10 wherein the resin is a random novolac resin.

13. A process as defined by claim 10 wherein the alkali metal is sodium.

14. A process as defined by claim 10 wherein, following the heating step, the article is rinsed with Water and dried at about 160 C. for a period of about 60 seconds.

References Cited UNITED STATES PATENTS 3,197,344 7/1965 Plaxton 148 -615 Z 2,976,176 3/1'961 Parks 117l61 L X 2,703,768 3/1955 Hall 1486.15 R 3,519,493 7/1970 Farone 1486.15 R 2,825,706 3/1958 Sanders 26014 3,108,083 10/1963 Laganis 260-172 X RALPH S. KENDALL, Primary Examiner C. WESTON, Assistant Examiner US. Cl. X.R.

1l7132 BF, 155 L 

